CALOMEL occurs in nature as the mineral horn-quicksilver, found as translucent tetragonal crystals with an adamantine lustre and whitish grey or brownish colour; it is mercurous chloride (mercury subchloride), Hal,. The chief localities are Idria, Obermoschel, Horowitz in Bavaria and Almaden in Spain. It was used in medicine as early as the i6th century under the names Draco mitigates, Manna metallorum, Aquila alba, Mer curius dulcis; later it became known as calomel, a name probably derived from the Greek KaXos, beautiful, and µEXas, black, in al lusion to its blackening by ammonia, or from KaXos and j. Xt, hon ey, from its sweet taste. It may be obtained by heating mercury in chlorine, or by reducing mercuric chloride (corrosive subli mate) with mercury or sulphurous acid. It is manufactured by heating a mixture of mercurous sulphate and common salt in iron retorts, and condensing the sublimed calomel in brick chambers. In the wet way it is obtained by precipitating a mercurous salt with hydrochloric acid. Calomel is a white powder which sub limes at a low red heat ; it is insoluble in water, alcohol and ether. Long continued boiling or prolonged digestion with water, dilute hydrochloric acid or solutions of alkaline chlorides convert it into mercuric chloride with deposition of mercury.
The molecular weight of mercurous chloride has given occasion for much discussion. E. Mitscherlich determined the vapour den sity to be 8.3 (air =1), corresponding to HgCl. The supporters of the formula pointed out that dissociation into mercury and mercuric chloride would give this value, since mercury is a monatomic element. A. Werner determined the molecular weight of mercurous chloride in pyridine solution, and obtained results pointing to the formula HgCl. However, the double formula, has been completely established by H. B. Baker, 1900, by vapour density determinations of the absolutely dry substance.
Calomel possesses certain special properties and uses in the pharmacology and therapeutics of mercury (q.v.). The specific value of mercurous chloride is that it exerts the valuable properties of mercuric chloride in the safest and least irritant manner, as the active salt is continuously and freshly generated in small quantities. Its pharmacopeia) preparations are the "Black wash," in which calomel and lime react to form mercurous oxide, a pill still known as "Plummer's pill," and an ointment. Externally the salt has not any particular advantage over other mercurial compounds, despite the existence of the officinal ointment. In ternally the salt is given in doses—for an adult of from one-half to five grains. It is an admirable aperient, acting especially on the upper part of the intestinal canal (duodenum and jejunum). It is well to follow a dose of calomel with a saline purgative a few hours afterwards. The special value of the drug as an aperient depends on its antiseptic power and its stimulation of the liver. The salt is often used in the treatment of syphilis, but is prob ably less useful than certain other mercurial compounds. Calomel or alternatively mercuro-salicyl arsenate is the active constituent of an ointment (unguentum prophylaxis) employed as a prophy lactic measure against syphilis.